The properties of many macroscopic structures depend in large part on their surface properties. For example, the rate of heat transfer between a structure and its surroundings depends on the ease with which radiative, conductive, and convective heat transfer occur between the surface of the structure and the surroundings. As another example, the strength of composite materials is often governed by the strength of the bond between the "internal" surfaces joining the different lamina. As yet another example, the rate of activity of a catalytic surface often depends on its surface area.
Efforts have been made to control the interaction of surfaces with their surroundings by painting, roughening, anodizing, hardening, plating, smoothing, and the like. In many cases, the resulting improvements in surface properties are relatively small.
One area where surface effects are important is the operation of gas turbines. The efficiency and power of a turbine increase as the maximum allowable gas inlet temperature increases. This allowable inlet temperature is a function of the composition of the turbine blades, and the balance of various modes of heat transfer into and out of the blade. Internal active cooling of turbine blades (a mode of heat removal), coupled with thermal barrier coatings on their surfaces limiting heat transfer into the blades), allows the blades to operate at a relatively low temperature in an environment hundreds of degrees higher. A reduction in the rate at which heat is transferred from the surrounding combustion gases to the blade would allow operation at higher temperatures and efficiencies.
Both the thermal efficiency and the power output of a turbine rise as the pressure ratio and the accompanying inlet temperature increase. For example, using estimates of turbine performance based on the Brayton cycle and standard cold air assumptions, a turbine receiving air at 300.degree. K with a compressor ratio of 13, and operating at a maximum turbine inlet temperature of 1400.degree. K has a thermal efficiency of 52% and a work output per kilogram of incoming air of 404 kJ/kg. The same turbine, with the same added heat/kg, but at a maximum turbine inlet temperature of 1600.degree. K can operate at a higher pressure ratio, will have a thermal efficiency of 63.5%, and will have work output per kilogram of incoming air of 494 kJ/kg.
Prior thermal barrier coatings (TBCs) have typically consisted of a ceramic thermal insulating layer, such as partially-stabilized zirconia (PSZ), bonded to a superalloy substrate by an oxidation-resistant alloy bond coat, such as NiCrAlY. ZrO.sub.2 is usually chosen as the ceramic material because the mismatch between its thermal expansion coefficient (a) and that of the metallic Ni-alloy substrate is relatively small. The bond coat serves several purposes: (1) it has an .alpha. between that of the substrate and that of PSZ, reducing the effects of .alpha. mismatch; (2) it provides oxidation resistance (PSZ is not a good barrier against oxidation); and (3) it promotes adhesion of the PSZ layer. See, e.g., J. Jedlinski, "Oxidation-Induced Degradation of Coatings on High Temperature Materials: An Overview," in N. Dahotre et al. (eds.), Elevated Temperature Coatings: Science and Technology I, pp. 75-83 (1995); J. Jue et al., "Characterization of Yttria and Rare Earth-Oxide Doped Zirconia Materials for High Temperature Applications," in N. Dahotre et al. (eds.), Elevated Temperature Coatings: Science and Technology I, pp. 125-134 (1995); and S. Lau et al., "Degradation Mechanisms of Ceramic Thermal Barrier Coatings in Corrosive Environments," in S. Singhal (ed.), Conf. Proc. 112th AIME, pp. 305-317 (1983).
Prior methods for modifying heat transfer have included radiation heat shields, heat fins, and painted, polished, or roughened surfaces. See, e.g., W. Rosenau et al., "Fins," Heat, Mass, and Momentum Transfer, pp. 106-109 (1961). No prior method for optimizing heat transfer between the surface of a structure and its surroundings has used microstructures. As discussed further below, I have discovered a technique for optimizing heat transfer by covering a surface with microstructures.
Prior methods of acoustic dampening have included the use of insulating layers; absorptive baffles; and "active," microprocessor-controlled cancellation of sound through the use of "complementary," out-of-phase sound waves. See, e.g., G. Diehl, "Machinery Sound Control," Machinery Acoustics, pp. 137-147 (1973). No prior method of acoustic dampening has used microstructures. As discussed further below, I have discovered a technique for acoustic dampening by covering a surface with microstructures.
No prior work on composite materials has used microstructures to improve bonding between layers of a composite. Prior methods of bonding laminates have generally used chemical bonding techniques. As discussed further below, I have discovered a technique for bonding layers of a laminate by using microstructures at the interface.
The LIGA process for making microstructures is well known in the art. See, e.g., E. Becker et al., "Fabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography, Galvanoforming, and Plastic Moulding (LIGA Process)," Microelectronic Engineering, vol. 4, pp. 35-56 (1986). The conventional LIGA process combines deep-etch X-ray lithography, electrodeposition, and polymer molding. Conventional LIGA is illustrated schematically in FIGS. 4(a) through 4(d). As illustrated in FIG. 4(a), an electrically conductive substrate 102 is glued or chemically bonded to a layer of a photoresist 104 such as polymethyl methacrylate ("PMMA") tens or hundreds of microns thick. The layer of resist is exposed to x-rays 106 through mask 108. Where mask 108 allows radiation 106 to pass, resist 104 degrades and becomes soluble in a developer. After development (FIG. 4(b)) the regions on substrate 102 that are no longer covered with resist 104 serve as initiation sites to electroform metal microstructures 110 (FIG. 4(c)). Following electroforming, removal of the remaining resist 104 produces a substrate covered with free-standing structures (FIG. 4(d)), which may then be used as a mold insert in forming a polymeric microstructure (not illustrated).
Prior work on microstructures (those whose dimensions are smaller than about 1 mm) has focused almost entirely on "microscopic" uses of microstructures. Little consideration has been given to "macroscopic" applications of microstructures, i.e., the use of microstructures to affect the interactions between macroscopic objects and their surroundings. No prior, general methods are known for manufacturing microstructures directly onto a metal surface of interest.